Multilayer ceramic substrates (MLC) are used in the production of electronic substrates and devices. The multilayer ceramic substrate typically includes alternating patterned metal layers sandwiched between ceramic layers. The patterned metal layers act as electrical conductors, whereas the ceramic layers act as a dielectric or insulating medium. For the purposes of interlayer interconnections, most of the ceramic layers have tiny holes or vias filled with an electrically conductive paste containing a low-resistance conductive material such as Ag, Ag/Pd alloy, Cu, or Au. The ceramic and metal layers are sintered to form vias, which provide the electrical connection between layers. In addition, the multilayer ceramic substrates may have termination pads for attaching semiconductor chips, connector leads, capacitors, resistors and the like.
The multilayer ceramic substrates are typically formed from ceramic green sheets in a layer-by-layer process that starts with a fired ceramic layer onto which alternating patterned layers of dielectric and metal conductive layers are sequentially deposited by a screening operation to form the multilayer ceramic structure. The layers can be sintered after each deposition or sintered after all the layers are deposited. Sintering at sufficiently high temperatures consolidates the metal, ceramic and usually glass particles into a dense, impervious monolith that provides mechanical support of the electrically continuous circuit traces formed within the ceramic dielectric. In practice, the layering process can be inefficient and requires the manufacturer to sequentially build the MLC structure from individual layers.
Another method to make multilayer ceramic substrates involves forming the dielectric into a tape by a casting process and punching the tape to form holes or xe2x80x9cviasxe2x80x9d into which a conductive metal paste is deposited along with metal traces that act as wiring. A number of these punched and metallized tapes are aligned, stacked and pressed into a laminate that is subsequently sintered. Such a methodology is a co-fired process in which the ceramic and metal powders are consolidated in essentially a single but often complex heating process. Similar to the layer by layer process, this type of process can be inefficient and requires the manufacturer to sequentially build the MLC structure from individual layers.
Traditional ceramic substrates are formed of aluminum oxide (Al2O3) which are typically bonded with a fused glass powder after sintering. Molybdenum and tungsten metal powders are commonly used as the electrical conductor since these materials provide high electrical conductivity while tolerating the high sintering temperatures without melting. Sintering such aluminum oxide ceramic substrates requires temperatures in excess of 1500xc2x0 C.
Current fabrication of MLC substrates requires each individual layer to be sequentially built to form the complete structure. Consequently, each layer is treated as a custom fabricated layer, which results in inefficiencies and extended cycle times to manufacture the complete multilayer ceramic substrate.
A process for forming a multilayer ceramic substrate includes forming a universal monolith and a custom monolith. The universal monolith includes a plurality of universal layers and at least one conductive pathway extending therethrough. The universal layers are fabricated using standard design rules and may further include redundant conductive pathways. A bottom surface of the universal monolith is adapted to electrically communicate with a first electrical component. The custom monolith includes a plurality of custom layers and a conductive pathway extending therethrough. The custom layers are fabricated using standard or advanced design rules. An upper surface of the custom monolith is adapted to electrically communicate with a second electrical component. The custom monolith is placed onto the universal monolith and each one of the conductive pathways in the custom monolith is aligned with a corresponding conductive pathway in the universal monolith. The custom monolith and the universal monolith are then bonded together to form the multilayer ceramic substrate, wherein each one of the conductive pathways in the custom monolith is in electrical communication with the corresponding conductive pathway in the universal monolith.
Other objects and a fuller understanding of the invention will be had from the accompanying drawings and detailed description that follows.